Mobile computing devices (such as smart phones, handheld computers, and tablet computers) are becoming increasingly more popular and offer an expanding array of capabilities. For example, today's mobile devices typically have the ability to remotely access the Internet, view a user's email accounts, capture images or video using built-in cameras, and store large amounts of user information. The user information may include, for example, personal information such as photos, files, and/or contact information. As a result of their compact design, mobile devices are often used in public environments and are more prone to being lost or stolen than their larger counterparts. Many mobile devices do not include a suitable mechanism for authenticating a user before allowing access to data or other functions offered by the device, making the user information on the mobile device vulnerable to misuse or theft. Furthermore, if an authentication mechanism is used on a mobile device, the mechanism typically comprises a password consisting of a small number of digits. Such an authentication mechanism is not robust, as the password can be easily observed or deduced given the relatively small number of possibilities.